Phoronids (Horseshoe Worms)

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Phylum Phoronida

Phoronida is a small phylum consisting of approx 15 species of worm-like creatures commonly known as phoronids or horseshoe worms. They are found in shallow temperate and tropical waters, rarely going deeper than 180 ft (54 m). Most species are tube dwellers, creating the tubes from a combination of their own secretions and surrounding sediment and shells. The tubes are usually anchored to rocks or shells even though phoronids are free to move around if they so chose, or the tubes may be buried in sand. Some species donít even make tubes, but instead bore into rock, living in the freshly-dug hole. One species is unusual in that it partially creates its tube and then intertwines it with the tube of a tube-dwelling sea anemone.

Phoronids share several basic features. They are typically elongated and worm shaped, but their bodies are not segmented. They are not very long, generally being 1/4 inch (6.25 mm) in length, although one species reaches 12 inches (30 cm).

The mouth is located at the top of the body and is surrounded by 18-500 (depending on the species and age) ciliated tentacles known as lophophores. These tentacles are usually the only part of the organism visible when it is in the tube, waving in the water to capture small particles of food. The food is drawn into the mouth and enters the digestive tract.

The digestive tract is perfectly suited for a tube-dwelling organism. It forms a U-shape, going from the mouth down to the far end of the body and then back up to an anus, located near the mouth. This feature is not present in true worms, and prevents the phoronid from releasing waste into the tube.

Phoronids can readily regenerate body parts such as lophophores. In some species, lophophores that have been cut from the body can regenerate a whole other body! Even though they can readily regenerate, asexual reproduction is rare, with only one species being known to do this on a regular basis. The other species are either divided into 2 sexes or are hermaphrodites that fertilize their own eggs.

One interesting aspect of phoronids is that they carry blood throughout their body in the form of red blood cells. These cells even contain haemoglobin, the oxygen-carrying molecule present in our own blood!

Since this phylum is so small, no class or order has been arranged. There are between 10-15 known species placed in one family: